The use of MOSFET type semiconductors to turn on electric devices, such as stepper motors and the like is described, for example, in U.S. Pat. No. 4,924,158 entitled “Motor Driver Protection Circuit”.
An application of such MOSFET devices for selective speed motor operation is found in U.S. Pat. No. 6,066,834 entitled “Speed Control Device for DC Motors”.
A further application of MOSFET devices for over current protection is described within U.S. Pat. No. 5,550,701 entitled “Power MOSFET with Over current and Over-Temperature Protection and Control Circuit Decoupled From Body Diode”.
When MOSFET devices are used in the continuous operation of such electrical devices, an N-CHANNEL MOSFET is usually employed due to the lower gate resistance of the N-CHANNEL MOSFET compared to a P-CHANNEL MOSFET. The use of such N-CHANNEL MOSFETS, however, requires additional circuitry to drive their gates above the high side supply.
A further deterrent to the use of the P-CHANNEL MOSFETS is the need for a level-shifting circuit component to insure that the source voltage does not exceed the gate voltage to prevent damage to the P-CHANNEL MOSFET.
One purpose of the instant invention is to provide a low-cost shifting circuit to maintain the gate of a P-CHANNEL MOSFET at a voltage less than maximum gate voltage to prevent damage to the P-CHANNEL MOSFET at a minimum increase in component cost. A further purpose is to provide high speed switching with minimum current draw.
A further purpose of the Invention is to incorporate both P-CHANNEL MOSFETs and N-CHANNEL MOSFETs advantageously within a solid state control circuit for operating electrical devices such as electric motors and the like.